Rename sysroot directories to be more clear

[jyn514]

These are effectively user facing because they're used for rustup link. So far this has worked reasonably well, but after #101691 stage0-sysroot will become user facing as well, at which point it will become extremely confusing that there's both that and stage0.

I suggest renaming stage0 to bootstrap-sysroot, keeping stage0-sysroot as is and renaming stage1 to stage1-sysroot.

cc @bjorn3 @Mark-Simulacrum

Originally posted by @jyn514 in #101691 (comment)

[jyn514]

Mentoring instructions: change the names in https://github.com/rust-lang/rust/blob/405f0c21608cb41aabc512eed73b2ff707391cab/src/bootstrap/compile.rs#L1107-L1111 and

rust/src/bootstrap/bootstrap.py

Line 600 in b05f97d

subdir = "stage0"

[chansuke]

@rustbot claim

[NSHU17]

@rustbot claim